The present invention relates to a process for separating nitrogen from a liquefied gas predominating in methane and containing a significant amount of nitrogen. More specifically, the present invention relates to the separation of nitrogen from a liquefied gas predominating in methane and containing a significant amount of nitrogen in conjunction with the liquefaction thereof.
While most natural gas predominates in methane, it can also contain significant amounts of C.sub.2, C.sub.3, C.sub.4 and C.sub.5 and higher molecular weight hydrocarbons. Where the gas is to be used as a fuel the C.sub.2 and higher molecular weight hydrocarbons are generally removed, to the extent practical, since these materials are generally of greater value for purposes other than as a gaseous heating fuel. For example, C.sub.2, C.sub.3 and C.sub.4 are valuable chemical intermediaries and the C.sub.3 and C.sub.4 hydrocarbons are of greater value when separated and utilized as liquefied petroleum gases (LPG). C.sub.5 and higher molecular weight hydrocarbons increase the heating value of natural gas but are normally removed, since they are valuable as blending stocks for motor fuels and for other purposes. In addition, failure to remove C.sub.5 and heavier hydrocarbons at an early stage can cause freezing problems in later stages of the process. However, in addition to these useful components, natural gas will in most cases also contain significant amounts of acid gases such as CO.sub.2 and H.sub. 2 S, water and nitrogen, all of which are considered impurities which reduce the heating value of the natural gas, cause other problems and are removed in most instances to the extent possible.
There are a number of valid reasons for the liquefaction of natural gas. For example, demand for the gas is seasonal and thus during certain periods demand is higher than normal, while during other periods demand is lower than normal. In order to be able to supply gas during periods of peak demand, it is customary to store gas at the area of use during periods of low demand for use during the periods of high demand. The most practical and economical method of storing natural gas in most instances is by the liquefaction of natural gas, since liquefaction reduces the volume of the gas to about 1/600 of the volume of the natural gas in its gaseous state. A highly significant increase in the liquefication of natural gas is for transport, particularly by ocean-going vessels, where the transport of natural gas in its gaseous state by pipeline is either impractical or impossible. In order to store or transport natural gas in its liquefied state, the temperature of the gas is reduced to about -258.degree. F. at atmospheric pressure.
In the liquefaction of natural gas it is customary to first remove acid gases such as CO.sub.2 and H.sub.2 S and then pass the gas through a dehydration system to remove water. The gas is then cooled by passing the same sequentially through a plurality of cooling stages, at successively lower temperatures, in which cooling is carried out by the expansion of compressed refrigerants in heat exchange with the gas to be liquefied. The refrigerants are derived either from the natural gas itself or supplied from an external source. One common practice is to utilize a series of successively lower boiling point refrigerants, such as propane or propylene followed by ethane or ethylene and finally methane. The refrigerants thus utilized are supplied in liquefied form by compression-refrigeration systems usually arranged in cascade fashion when a plurality of refrigerants are utilized in sequence. However, a more efficient process compresses the gas to a high pressure, if it is not already at a sufficiently high pressure, prior to cooling and substitutes a series of pressure reduction or flash stages for the methane cycle. This not only has the advantage of further cooling the gas as it is being reduced in pressure to essentially atmospheric pressure but the refrigeration potential of the flashed gases which result from the pressure reduction steps can be utilized to further cool the liquefied gas and then be recycled back to the main gas stream. For purposes of such recycle the gas is generally compressed to a pressure near the pressure of the main gas stream at the point at which the recycle gas is recombined therewith and in some instances cooled to a temperature near the temperature of the main gas stream at such point of recombination. In the last mentioned natural gas liquefaction systems it is conventional practice to remove the nitrogen after the natural gas has been liquefied by either passing the gas through a fractionation column, usually referred to as a nitrogen removal column, or in the first stage of the multiple stage pressure reduction cycle. In both instances the vapor phase containing the major portion of the nitrogen is still primarily methane and therefore is utilized as a fuel for use in operating compressors, turbines and the like in the liquefaction plant. In any event, such conventional nitrogen separation systems do not remove a sufficient volume of the nitrogen, particularly where the gas has a higher nitrogen content, and are inefficient energywise. For example, in addition to reducing the heating value of the liquefied natural gas, the retention of too much nitrogen in the liquefied natural gas will result in increasing the horsepower requirements for compressing the recycle gas and to some extent the horsepower requirements for compressing the refrigerants utilized to liquefy the gas. Such conventional nitrogen separation systems also fail to utilize the full refrigeration potential of the flashed gases in some instances.